The measurement of current distribution in superconducting tape.: Comparison of destructive and non-destructive methods

Abstract

The question whether the current in superconducting tapes flows homogeneously or is distributed in some way across the width, between its edges, can be answered in several ways. If we are interested in the critical current distribution only, the destructive method can be applied. The tape can be cut longitudinally into several sub-tapes and the critical current can be measured in each sub-tape. The principle of the method is trivial and straightforward. Other modifications of destructive method exist, e.g. gradual brushing of the tape from one side and looking for a change in the overall critical current in the rest of the tape. The disadvantage of both methods is rough mechanical interaction with the tape, which can modify its transport properties and inability to repeat measurements on the same piece of initial sample after changing some of the external parameters. The magnetic knife method is an example of a non-destructive approach. Still it is from principle limited to the critical current distribution only. Current distribution in general, without limitation to the critical one, can be achieved by measuring the self-magnetic-field of the tape and solving the inverse problem for corresponding current distribution. An improved, unique procedure of calculation is described here in detail. The results for special case of current equal to the critical current, i.e. the critical current distribution across the tape width, are compared with the results of the destructive method applied on the same tape. The possibility to check the method by making comparison is the only reason for limitation to (zero field) critical current. Any current distribution, at any phase of the tape transport current during AC cycle, can be determined by the non-destructive method. As an example, frozen currents distribution, after ramping transport current up to I c and afterwards down to zero current, are shown.